1. Field of the Invention
The present invention relates to methods for manufacturing photomask blanks and photomasks suitable for use in the microfabrication of semiconductor integrated circuits, charge coupled devices (CCD), liquid crystal display (LCD) color filters, magnetic heads and the like.
2. Prior Art
Photomasks are used in a broad range of applications including the manufacture of semiconductor integrated circuits as typified by ICs and LSIs. The photomask basically comprises a light-transmissive substrate and a desired pattern of light-shielding film of chromium or the like formed thereon. The market demand for ever higher levels of integration in semiconductor integrated circuits has led to a rapid reduction in the minimum feature size of photomask patterns. Such miniaturization has been achieved in part by the use of shorter wavelength exposure light.
Although exposure using shorter wavelength light does improve resolution, it has undesirable effects, such as reducing the focal depth, lowering process stability and adversely impacting product yield.
One pattern transfer technique that has been effective for resolving such problems is phase shifting. This involves the use of a phase shift mask as the mask for transferring microscopic patterns.
The phase shift mask is constructed such that the phase of light changes with locations where light is transmitted, whereby a fine pattern is formed utilizing the interference between lights of different phases. The phase shift mask is generally composed of a substrate on which a phase shift film is patterned. The mask has both exposed substrate areas (first light-transmitting areas) on which there is no phase shift film, and phase shifters (second light-transmitting areas) that form a pattern region on the mask. The phase shift mask improves the contrast of a transferred image by providing a phase difference of 180 degrees between light passing through the pattern region and light passing through the non-pattern region, and utilizing the destructive interference of light at the boundary regions of the pattern to set the light intensity in the areas of interference to zero. The use of phase shifting also makes it possible to increase the focal depth at the necessary resolution. Hence, compared with a conventional mask having an ordinary light-exposure pattern made of chromium film, the phase shift mask can improve resolution and increase the margin of the exposure process.
For practical purposes, the phase shift masks can be broadly categorized, according to the light-transmitting characteristics of the phase shifter, as either completely transmitting phase shift masks or halftone phase shift masks. Completely transmitting phase shift masks are masks in which the phase shifter has the same light transmittance as the substrate, and which are thus transparent to light of the exposure wavelength. In halftone phase shift masks, the phase shifter has a light transmittance that ranges from about several percent to several tens of percent the transmittance of exposed substrate areas.
The halftone phase shift masks proposed thus far are halftone phase shift masks of the single-layer type which are simple in structure. Single-layer halftone phase shift masks known to the art include those described in JP-A 7-140635 which have a phase shifting film composed of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON).
Such phase shift masks are manufactured by lithographically patterning phase shift mask blanks. The lithography involves the step of applying a resist onto a phase shift mask blank, irradiating selected portions of the resist with electron beams or ultraviolet radiation, developing the resist, and etching desired portions of the phase shift film. Thereafter, the resist film is stripped, leaving a phase shift mask.
In a photomask blank like the phase shift mask blank discussed above, a film like the phase shift film is generally formed on a substrate by sputtering. Stresses are induced in the film, by which the substrate is distorted. The resulting photomask blank is thus warped. If a photomask is manufactured through patterning of such a photomask blank, the warpage of the substrate is locally resumed to the original state prior to film formation because the film is partially removed by patterning. The resulting substrate has varying degrees of flatness. These changes introduce positional shifts between the mask blank during the pattern exposure and the actually finished mask. Such positional shifts have a larger influence as the mask pattern becomes finer, and the warpage sometimes cause focal shifts.
In manufacturing a photomask from a photomask blank, wash liquids are used to remove surface stains and foreign matter. The wash liquids used include acids such as sulfuric acid, and alkali solutions such as aqueous ammonia and a mixture of aqueous ammonia and aqueous hydrogen peroxide. Most films used in photomask blanks and photomasks are not enough resistant to such chemicals as acids and alkalis, and in particular, the phase shift film undergoes an undesirable change of phase difference by alkali treatment.